1. The Field of the Invention
This invention relates to systems and methods for automatically announcing the position of one or more mobile craft to a receiver positioned at a remote location. More particularly, the present invention relates to systems and methods for tracking the positions of a plurality of mobile craft so as to provide assistance in navigation, surveillance, emergency location and collision avoidance.
2. The Prior Art
Throughout recorded history there has been an ever increasing need for more precise navigational aids. Through the years, the time keeping and position determining devices used for navigation have evolved from sun dials and sextants to sophisticated digital electronic systems capable of providing nearly pinpoint positioning accuracy almost anywhere in the world.
In today's transportation and communications environment, many devices exist that provide specific navigational aid to the operators of air, sea, and land craft. Navigational aids such as the Global Positioning Satellite system (GPS), the Long Range Navigation system (LORAN), and other navigational aids commonly referred to by such various acronyms as: VOR-DME, VOR-TACAN, DECCA, OMEGA, NDB, ILS, MLS, and ADF, are used by craft operators, particularly aircraft pilots, to determine the position of their craft in one, two, or three planes in space. For example, an operator of an aircraft may only desire to know the position of the craft above or below the surface of the earth (altitude or depth) or may desire to know only the position of the craft relative to a fixed terrestrial reference system (latitude and longitude) or the operator may desire to know the craft's position in all three planes in space.
One of the purposes of the various available navigational aids is to allow operators of craft, in particular air and water craft, to avoid collisions between their craft. As is well known, mid-air collisions of aircraft almost always result in disastrous loss of life and property. In an effort to avoid collisions between craft various systems have been implemented in the air and marine transportation industries.
In the United States, government agencies dealing with the air transportation industry have recognized a need to prevent mid-air collisions. In an effort to reduce the occurrence of mid-air collisions, U.S. government agencies have mandated that by the end of 1991 all commercial aircraft with thirty or more seats be equipped with collision avoidance equipment. Several types of collision avoidance equipment have been devised and are classified as either "active" or "passive" systems. These systems are commonly designated as Traffic Alert and Collision Avoidance Systems, also referred to as TCAS.
TCAS has been designed as a primary collision avoidance system for commercial aircraft and has received the United States Federal Aviation Administration's approval. An "active" TCAS system (TCAS II or III) provides the capability to interrogate other nearby transponder-equipped aircraft to elicit a responsive reply, while "passive" TCAS systems (TCAS I) simply eavesdrop on nearby interrogation replies from other transponders. In order to detect a "bearing," a TCAS system requires an expensive directional antenna.
The TCAS method is dependant upon either a secondary or beacon surveillance radar system, or other transponder (S) type interrogations in order to elicit a responsive reply. It is also transponder dependant, meaning that any aircraft not equipped with a transponder will not be detected, and any transponder not coupled to an altitude encoder will not deliver altitude information.
Recently, serious questions have been raised about how "safe" TCAS is in practice. TCAS operates in a narrow band of Air Traffic Control (ATC) radio frequencies in the microwave region of the spectrum. Thus, because of the number of interrogation requests and replies elicited, and the amount of information needed to be processed both TCAS and the ATC system may be "overwhelmed," and their operation sufficiently degraded to the point of "saturation," where the number of aircraft under the control of ATC exceeds the capacity of the system. Already, computer overloads and radar shutdowns have occurred at some busy locations. There is also the danger of a malfunction occurring, such as an item of interrogation equipment becoming stuck in the transmit mode, thereby "locking up" the entire system.
The high cost of the on board TCAS equipment makes its installation prohibitively expensive to most aircraft other than commercial aircraft. Moreover, TCAS III has not yet been fully developed. Even further, TCAS II and III are designed with the commercial aviation market in mind, but of the 215,926 active aircraft registered in the United States in 1988, the commercial aviation fleet only amounted to about three percent (3%) of the total. Thus, it is apparent that improved collision avoidance systems need to become more accessible to a much larger portion of the aviation industry, as well as to land and sea-based craft.
In view of these drawbacks and difficulties it would be an advance in the art to provide a complete traffic control system not requiring radar, and which provides a system and method for announcing the position of a craft to a remote receiver such that the position of the craft can be continuously monitored and collisions with other craft avoided. It would also be an advance in the art to provide a system and method for collision avoidance which may be used in addition to, and without interference with, preexisting collision avoidance systems and which can be economically implemented in the general aviation and marine industries so that, for example, with a single frequency select switch seagoing vessels could monitor air or land-based traffic and vice versa.
It would be a further advance in the art to provide a system and method which allows the operator of a first craft to monitor the position of a plurality of other craft within a predetermined range of the first craft and to alert the operator of a craft of a potential collision between craft as early as possible. It would be yet another advance in the art to provide a system and method for announcing the position of craft to other craft which are within a predetermined range which is reliable, does not distract an operator of a craft from other duties, and provides short and long range navigational assistance to the operator of a craft.
It would be a still further advance in the art to provide a system and method of tracking the position of one or more craft within a predetermined range of a location anywhere on the earth such as at a remote airstrip, or beneath the surface of the ocean, or in space. It would be an even further advance in the art to provide a collision avoidance system which does not become saturated in areas of heavy traffic and which is capable of assisting with the landing or mooring of craft operating under poor visibility conditions. Still further, it would be an advance in the art to provide a system and method for tracking the position of a number of craft, for example a fleet of land craft which are carrying out tasks such as delivery of goods or people or other tasks.